Acid-base equilibria equilibrium expression

C18-0040. List all the types of calculations described in Chapter 18 in which acid-base equilibrium expressions play a role. 

Taking into consideration the acid-base dissociation equilibrium reaction of the ligand, one can obtain an expression for [L" ] and rewrite equation (61) as follows ... 

Generally, it has been found that the organic acids and bases do exist in aqueous solution as equilibrium mixtures of their respective neutral as well as ionic forms. Thus, these neutral and ionic forms may not have the same identical partition coefficients in a second solvent therefore, the quantity of a substance being extracted solely depends upon the position of the acid-base equilibrium and ultimately upon the pH of the resulting solution. Hence, extraction coefficient (E) may be defined as the ratio of the concentrations of the substance in all its forms in the two respective phases in the presence of equilibria and it can be expressed as follows ... 

The general form of Equation 8.114, which relates kinetics to complexation equilibrium, becomes identical to that presented in Equation 8.93 for acid-base equilibrium, once the latter is expressed in terms of protonation, that is, Kp = 1 /K.A. In that case, Equation 8.93 is replaced by... 

In addition, the acid/base equilibrium of trifluoroacetic acid must be established, which is expressed in Equations 4.13 and 4.14 ... 

Water is omitted from the expression because its concentration is not likely to be affected by this equilibrium. When the equilibrium constant, Kc, is written for an acid-base equilibrium, it is known as the acid-dissociation constant, Ka. The generic equation for the... 

Such considerations led to a more general definition of acids and bases, which was proposed independently by J. N. Brpnsted and T. M. Lowry in 1923. They defined acid as any substance (in either the molecular or the ionic state) which donates protons (H+), and a base as any substance (molecular or ionic) which accepts protons. Denoting the acid by A and the base by B, the acid-base equilibrium can be expressed as... 

The control variable in any acid-base equilibrium is pH hence it is desirable to represent graphically the equilibrium relationships of all species as functions of pH. For any value of pH the unknowns in the present example are of course [HA] and [A ], each of which may now be expressed in terms of the known quantities Cj and [H j by combining equations 41 and 42 as follows ... 

Thus, for every [H ], seven equations have to be solved simultaneously in order to compute the relative concentrations of each species present. Five mass laws (four stability expressions for the four different amine complexes and the acid-base equilibrium of NH4 -NH3) and two concentration conditions make up the seven equations. As concentration conditions one can formulate equations defining Cut- and NH3/. 

Scheme 3.1 is not a realistic expression, however, since protons are too reactive to exist independently and are rapidly taken up by the solvent. The proton-accepting entity, hy the Lowry-Bronsted definition, is a base, and the product formed when the proton has heen accepted by the solvent is an acid. Thus a second acid-base equilibrium occurs when the solvent accepts the proton, and this may be represented by... 

The radiolysis of water leads to the formation of the three radical species OH (hydroxyl radical), H (hydrogen atom) and e q" (hydrated electron), within the nanosecond time scale [3]. In aerated medium (that is in the presence of dioxygen, Oj concentration dissolved in water being 2 x 10 mol I ), the free radicals H and e are replaced by HO2 and Oj radical species, respectively, which are related by an acid/base equilibrium (pK (HO27O2 ) = 4.8).The radiolytic yields (G-values expressed in moles per Joule) of each radical species are well known 2.8 x 10 mol J and 3.4 x 10 mol J , respectively for OH and O2 free radicals at pH = 7 (Chapter 1) [3]. 

For an acid capable of ionizing into a univalent anion, and y will be approximately equal, and y, will be approximately equal to one. Writing the concentration-based equilibrium constant expression as ... 

Ka.concentration-based equilibrium constant expression of an acid... 

Let us consider the behaviour of the E-pd dependence in the section of the saturated solution of the studied oxide. At first, it should be noted that the acid-base equilibrium (3.6.1) in this section is described by the solubility product of the oxide (2.4.14), which can be expressed via the experimentally determined values in such a manner... 

Next, we need to consider the curves for [HOAc] and [OAc ] you should refer to Figure 7.3 during the following discussion. In strongly acid solutions, the dissociation of HOAc is suppressed and HOAc is the major form. Since Choac = 1.0 X 10" M, then in acid solutions [HOAc] is essentially constant at 1.0 X 10" M. Conversely, we know that in alkaline solutions [HOAc] becomes very small due to dissociation and that [OAc ] = Choao- For alkaline solutions, then, we can rearrange the acid-base equilibrium expression (substituting Choac for [OAc ]) to... 

Complete understanding of the shape of the curves in Figure 7.5 requires a kinetic expression somewhat more complicated than we wish to deal with here. However, the nature of the extremities of the curve can be understood on the basis of qualitative arguments. The rate decreases with pH in the acidic region because formation of the zwitterionic tetrahedral intermediate TI+ - is required for expulsion of the amine (Step 5). The concentration of the zwitterionic species decreases with increasing acidity, since its concentration is governed by an acid-base equilibrium. 

On the other hand, this acid-base equilibrium can be expressed in terms of an acid dissociation constant , Ka. ... 

Just as for weak acids, the equilibrium constant expression is written without including a concentration term for pure water. The resulting equilibrium constant is called a base ionization constant, Ki, ... 

Any quantitative measure of the acidity of organic acids or bases involves measuring the equilibrium concentrations of the various components in an acid-base equilibrium. The strength of an acid is then expressed by an equilibrium constant. The dissociation (ionization) of acetic acid in water is given by the following equation ... 

To find the pH of a solution of a molecular weak base, we use an approach very similar to that for a weak acid write the equilibrium expression, set up a reaction table to find make the usual assumptions, and then solve for [OH ]. The... 

Relative Concentrations of Buffer Components A buffer works because large amounts of the acidic (HA) and basic (A ) components consume small amounts of added OH or Hfi, respectively. Consider what happens to [H3O+] in a solution with high [CH3COOH] and high [CH3COO ] when we add small amounts of strong acid or base. The equilibrium expression for HA dissociation is... 

For a total concentration in chromium (expressed in atom-grams) higher than 10-1-68 [jioi/L on the one hand and with a pH value higher than 0.75 on the other hand, chromium at oxidation state +VI is either as dichromate ions Cr207 or as chromate ions Cr04 depending on the pH value (see Fig. 20.2). Both species are linked by the acid-base equilibrium ... 

C. Concentrations were in moles per 1000 g of liquid HF, i.e., molalities. The specific conductance L was derived from Lmeasured l-soivent- According to these authors, this expression did not represent the actual relationship because of interaction between the solvent and solute. A large part of the solvent conductance was attributed to traces of water and salts. Water was described as a strong electrolyte in liquid hydrogen fluoride, and also as a strong base therefore, it will interfere with the acid-base equilibrium under examination. 

Several types of reactions are commonly used in analytical procedures, either in preparing samples for analysis or during the analysis itself. The most important of these are precipitation reactions, acid-base reactions, complexation reactions, and oxidation-reduction reactions. In this section we review these reactions and their equilibrium constant expressions. 

Many organic reactions involve acid concentrations considerably higher than can be accurately measured on the pH scale, which applies to relatively dilute aqueous solutions. It is not difficult to prepare solutions in which the formal proton concentration is 10 M or more, but these formal concentrations are not a suitable measure of the activity of protons in such solutions. For this reason, it has been necessaiy to develop acidity functions to measure the proton-donating strength of concentrated acidic solutions. The activity of the hydrogen ion (solvated proton) can be related to the extent of protonation of a series of bases by the equilibrium expression for the protonation reaction. 

To use the above expression for measuring the strength of an acid, a standard acid-base pair, say A2-B2, must be chosen, and it is usually convenient to refer acid-base strength to the solvent. In water the acid-base pair H30 + -H20 is taken as the standard. The equilibrium defining acids is therefore ... 

Case 3. Salt of a weak acid and a weak base. The hydrolytic equilibrium is expressed by the equation ... 

For (acid + base) reactions, expressions can be written for the equilibrium constant that involve activity coefficients as well. For example, for the reactions... 

To express the relative strengths of an acid and its conjugate base (a conjugate acid-base pair ), we consider the special case of the ammonia proton transfer equilibrium, reaction C, for which the basicity constant was given earlier (Kb = [NH4+l[OH ]/ NH3]). Now let s consider the proton transfer equilibrium of ammonia s conjugate acid, NH4+, in water ... 

We calculate the pH of solutions of weak bases in the same way as we calculate the pH of solutions of weak acids—by using an equilibrium table. The protonation equilibrium is given in Eq. 9. To calculate the pH of the solution, we first calculate the concentration of OH ions at equilibrium, express that concentration as pOH, and then calculate the pH at 25°C from the relation pH + pOH = 14.00. For very weak or very dilute bases, the autoprotolysis of water must be taken into consideration. 

STRATEGY Because NH4+ is a weak acid and Cl- is neutral, we expect pH < 7. We treat the solution as that of a weak acid, using an equilibrium table as in Toolbox 10.1 to calculate the composition and hence the pH. First, write the chemical equation for proton transfer to water and the expression for Ca. Obtain the value of Ka from Kh for the conjugate base by using K, = KxJKh (Eq. 11a). The initial concentration of the acidic cation is equal to the concentration of the cation that the salt would produce if the salt were fully dissociated and the cation retained all its acidic protons. The initial concentrations of its conjugate base and H30+ are assumed to be zero. 

Table 16-2 List of input components for the simplest case of the acid-base balance of unpolluted marine clouds. Also shown are the mass conservation statements, chemical equilibrium expressions and constants, and the requirement for charge balance...

C16-0105. Write the equilibrium reaction and equilibrium constant expression for each of the following processes (a) Trimethylamine, (CH3)3 N, a weak base, is added to water, (b) Hydrofluoric acid, HF, a weak acid, is added to water, (c) Solid calcium sulfate, CaSOq, a sparingly soluble salt, is added to water. 

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